1. Field of the Invention
This invention relates generally to transceivers, and, more particularly, to a mixer for use in a transceiver.
2. Description of the Related Art
The increasing demand for portable wireless communication devices having low price, weight and size and improved capabilities is prompting research in new IC (integrated circuit) technologies, circuit configurations and transceiver architectures. Transceiver implementations for wide band systems comprising direct conversion mixers are known and meet the above mentioned requirements better than architectures based on the widely used super-heterodyne principle.
At the transmitter stage of the transceiver, direct conversion mixers are used to up-convert a baseband analog or digital signal to an RF (radio frequency) signal for ease of transmission. At the receiver stage, direct conversion mixers are used to down-convert a received RF signal to baseband for ease of signal processing. Therefore, no high-Q filters and high-Q image rejection filters are necessary for image rejection and IF (intermediate frequency) filtering. Generally, it is difficult to integrate high-Q filters. Such receivers are also called zero-IF receivers, since the wanted signal is directly down-converted to the base-band and the IF is chosen to be zero. The mixers used therein commutate the amplified RF signal with the LO (local oscillator) signal. For example, in the often-used bipolar mixer based on the Gilbert analog multiplier, a current-mode commutation is performed. This multiplier is also called Gilbert cell, which is a cross-coupled differential amplifier.
For such direct conversion topologies, several problems like carrier leakage, second order intermodulation, and interference between local oscillator and RF signals exist.
Especially direct conversion receivers require a high degree of linearity of the mixer stage, since second order spurious products fall directly into the obtained baseband frequency and disturb the desired signal. The main reason for such second order mixer non-linearity is based on signal cross talk between the input signals of the mixer. Generally, this will lead to signal self-mixing effects causing DC (direct current) offset. However, this DC offset is not constant.
Further problems in state of the art transceiver architectures are pulling effects. In principal, such effects can be prevented by isolating the VCO (Voltage Controlled Oscillator) generating the LO signal from all other signals. However, isolation is a problem for architectures where the VCO is operating at the transmit frequency, i.e., FM (frequency modulation) systems using direct modulation of the VCO or a direct up-conversion principle. In such transceiver architectures, the power amplifier (PA) or a power pre-amplifier generates strong signals on the chip at the same frequency the on-chip VCO is operating at. The same problem occurs if strong signals are applied to the Rx (receive) input. VCO pulling is caused by non-perfect isolation, i.e., in transceiver topologies where the VCO is running at the same frequency as the Tx (transmit) output and the Rx input are operated. In modern transceiver architectures it is desirable to reduce such effects.
In state of the art receivers, the incoming RF signal is multiplied by a sinusoid signal derived from a local oscillator (LO signal). Both signals may be represented by a voltage or a current. The mixer performing the multiplication of both signals comprises two inputs that are practically not completely decoupled. Therefore, in addition to the wanted signal, each mixer input signal additionally contains a smaller cross-coupled portion of the other signal. Due to the multiplying property of the mixer, the output signal contains spurious signals, which are proportional to the power of the received signal centered around DC. These spurious signals are especially disadvantageous for the direct conversion principle, since the desired down-converted RF signal is also centered at a frequency of f=0.
The present invention solves, or at least reduces, some or all of the aforementioned problems.